1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device integrated with a drive circuit in which the drive circuit is formed on a substrate where a pixel electrode is also formed.
2. Description of the Prior Art
Liquid crystal display devices which are of high density, high capacity, high function and high definition are being developed for practical use in these years. Among these liquid crystal devices, an active matrix type liquid crystal display device which has a thin-film transistor (TFT) as drive element is often used because high contrast display is obtained without any crosstalk between continuously disposed pixel electrodes, transmission display can be made, and a large size screen is made with ease.
As shown in FIG. 13, this display device comprises a first substrate 11 made of glass and a second substrate 12 made of glass which is disposed to oppose the first substrate 11 with a gap between them, and has a gap 15 for filling a liquid crystal layer 14 therein which is formed by placing a sealing agent on the peripheries of the substrates.
The area where the sealing agent 13 is disposed is a seal area (120), and the area surrounded by the seal area (120) comprises an image display area (100) and a display outside area (110) on the outer periphery of the image display area (100).
The areas other than the image display area, namely a seal area (120), a display outside area (110) and a seal area outer edge (140), are called a frame area (130).
A plurality of pixel electrodes are disposed in the form of matrix on the image display area (100) of the first substrate 11, and drive elements are connected to the respective electrodes.
Conventional active matrix substrates for the liquid crystal display device theretofore developed have an amorphous silicon thin-film transistor (hereinafter referred to as a-Si TFT) or a polysilicon thin-film transistor (hereinafter referred to as p-Si TFT) and have been placed on the market.
Among them, the p-Si TFT has a high-electron mobility therein, its drive element size can be made small as compared with the a-Si TFT, an aperture ratio of its pixel electrode can be improved, and its drive circuit is integrally formed on the active matrix substrate by the same p-Si TFT. Therefore, a drive IC which is separately produced and mounted is not needed, labor for its mounting process can be saved, and the device can be produced at a lower cost. Besides, since the frame area around the display area on the active matrix substrate can be made small, its development is promoted.
On the other hand, the liquid crystal display device needs the liquid crystal layer which has a uniform thickness through the entire area of the image display area for uniform displaying, and it is essential that two substrates have the same interval. Therefore, the gap between the substrates is controlled by means of a spacer by:
(1) dispersing microspherical spacer within the image display area, and PA0 (2) mixing the spherical spacer or glass fiber having substantially the similar diameter into the sealing material for sealing the two substrates.
In the structure that the p-Si TFT is used to form the drive element of the pixel electrode and the same p-Si TFT is also used to integrally form the drive circuit of the drive element on the same electrode substrate, the position where the drive circuit is disposed with respect to the seal area is a problem.
The structure disposing the driving circuit on the outer periphery of the seal area has the glass substrate with a size larger than the seal area so to protrude from the seal area to secure the disposition area where the drive circuit is formed. Therefore, it is hard to make the display cell small.
Japanese Patent Laid-Open Publication No. Sho 62-251723 indicates that reliability is questionable because the liquid crystal layer is disposed on the drive circuit.
Therefore, it is proposed to dispose the drive circuit on the seal area (U.S. Pat. No. 5,148,301). This structure effectively uses the seal area and is suitable to make the liquid crystal cell small. But, it has a disadvantage that a yield in production is lowered because the spacer for controlling the cell thickness is used as the sealing agent to be disposed on the drive circuit.
Furthermore, where the drive circuit is disposed on a single row side and a single column side on the outer periphery of the display area of a rectangular active matrix substrate, when a sealing means is disposed on the drive circuit of the outer periphery of the display area to surround it, the cell thickness restricted by the spacer, e.g., the glass fiber, is different for a height of the drive circuit between a side where the drive circuit is disposed and a side where no drive circuit is disposed to oppose the former drive circuit, the gap between the substrates becomes uneven, and display becomes defective. In this connection, Japanese Patent Publication No. Hei 2-242230 proposes to dispose a pseudo drive circuit pattern having the same height as the drive circuit on the side where no drive circuit is disposed. By forming the pseudo drive circuit pattern, variations in cell thickness can be lowered even when the sealing means is disposed on the drive circuit. But, a yield of the drive circuit is lowered similarly as in the above case when a cell thickness controlling member such as glass fiber is used.
In a conventional liquid crystal display device integral with the drive circuit using the p-Si TFT as drive element, as to the seal area of the sealing agent for bonding the active matrix substrate and the opposed substrate, the drive circuit on the active matrix substrate is disposed on the frame area of the substrate or the outer periphery of the drive circuit.
Accordingly, even though the drive circuit is integrally formed with the active matrix substrate, the frame area around the display area becomes large. Besides, a liquid crystal display device using them is prevented from being made small, and a plurality of liquid crystal display panels cannot be arranged in close proximity to one another, so that a folding-type liquid crystal display device or a large-size liquid crystal display device cannot be made. With the structure having the drive circuit disposed on the seal area, the drive circuit is damaged by the glass fiber contained in the sealing agent to restrict the cell thickness, or the cell thickness becomes uneven depending on the presence or not of the drive circuit. Besides, since the glass fiber contained in the sealing agent is rigid and does not deform, a lead line of the signal line or scanning line running through the seal area to reach from the display area to the drive circuit might be broken by being compressed by the glass fiber, resulting in a failure.